Fix overlap check for ParamSpec types (#18040)

ParamSpec types can match arbitrary parameter types, so treat them as
having 'object' as the upper bound.

This also fixes issues with filtering of overload items based on self
type.

Fixes #18036.

mypy/meet.py

@@ -223,7 +223,11 @@ def get_possible_variants(typ: Type) -> list[Type]:
         else:
             return [typ.upper_bound]
     elif isinstance(typ, ParamSpecType):
-        return [typ.upper_bound]
+        # Extract 'object' from the final mro item
+        upper_bound = get_proper_type(typ.upper_bound)
+        if isinstance(upper_bound, Instance):
+            return [Instance(upper_bound.type.mro[-1], [])]
+        return [AnyType(TypeOfAny.implementation_artifact)]
     elif isinstance(typ, TypeVarTupleType):
         return [typ.upper_bound]
     elif isinstance(typ, UnionType):

test-data/unit/check-selftype.test

@@ -2176,3 +2176,41 @@ class count:
 def foo(x: Union[range[int], count]) -> None:
     for item in x:
         reveal_type(item)  # N: Revealed type is "builtins.int"
+
+[case testGenericDescriptorWithSelfTypeAnnotationsAndOverloads]
+from __future__ import annotations
+from typing import Any, overload, Callable, TypeVar, Generic, ParamSpec
+from typing_extensions import Concatenate
+
+C = TypeVar("C", bound=Callable[..., Any])
+S = TypeVar("S")
+P = ParamSpec("P")
+R = TypeVar("R")
+
+class Descriptor(Generic[C]):
+    def __init__(self, impl: C) -> None: ...
+
+    @overload
+    def __get__(
+        self: Descriptor[C], instance: None, owner: type | None
+    ) -> Descriptor[C]: ...
+
+    @overload
+    def __get__(
+        self: Descriptor[Callable[Concatenate[S, P], R]], instance: S, owner: type | None,
+    ) -> Callable[P, R]: ...
+
+    def __get__(self, *args, **kwargs): ...
+
+class Test:
+    @Descriptor
+    def method(self, foo: int, bar: str) -> bytes: ...
+
+reveal_type(Test().method) # N: Revealed type is "def (foo: builtins.int, bar: builtins.str) -> builtins.bytes"
+
+class Test2:
+    @Descriptor
+    def method(self, foo: int, *, bar: str) -> bytes: ...
+
+reveal_type(Test2().method) # N: Revealed type is "def (foo: builtins.int, *, bar: builtins.str) -> builtins.bytes"
+[builtins fixtures/tuple.pyi]